Experimental Design Sixty subjects between the age of 30 to 55 with absence of insulin resistance will be recruited in the study. Thirty of the adults will be with normal body mass index (BMI; 15 males and 15 females, 18.5 kg∙m-2 < BMI ≤25 kg∙m-2) and thirty overweight or obese adults (15 males and 15 females, 28 kg∙m-2 ≤ BMI ≤35 kg∙m-2) will be recruited to participate in the study. Sample size estimation showed that 60 subject will provide power of 0.8 with alpha level set at 0.05. The effect of vasopressin on glucose metabolism will be studied via osmotic stimulation of vasopressin (AVP) utilizing hypertonic saline infusion followed by an oral glucose tolerance test. Each subject will perform two identical experiments, differing only in the sodium chloride (NaCl) content of the infusion (isotonic vs. hypertonic saline). All female subjects will perform both trials during their early follicular phase, approximately 2-6 days post menses onset to ensure low endogenous levels of estrogen and progesterone. After resting in a seated position for 30 min, subjects will be infused intravenously either 3% NaCl (Hypertonic saline, hence HYPER) or 0.9 % NaCl (Isotonic saline, hence ISO) over a 120 min period at an infusion rate 0.1 ml/min/kg of body weight in a single-blind fashion and in counterbalanced order. This hypertonic saline infusion increases plasma osmolality from 285 to at least 300 mmol/kg. From a separate venous catheter, blood samples will be taken every 30 min. Following infusion, subjects will rest for an equilibration period of 30 min before they start a 4-h oral glucose tolerance test.